As is well known in the industry, crude oil contains heteroatom contaminants including, but not limited to, sulfur, nitrogen, phosphorus, nickel, vanadium, and iron and acidic oxygenates in quantities that negatively impact the refinery processing of the crude oil fractions. Light crude oils or condensates contain heteroatoms in concentrations as low as 0.001 wt %. In contrast, heavy crude oils contain heteroatoms as high as 5-7 wt %. The heteroatom content of crude oil increases with increasing boiling point and the heteroatom content increases with decreasing API gravity. These contaminants must be removed during refining operations to meet the environmental regulations for the final product specifications (e.g., gasoline, diesel, fuel oil) or to prevent the contaminants from decreasing catalyst activity, selectivity, and lifetime in downstream refining operations. Contaminants such as sulfur, nitrogen, phosphorus, nickel, vanadium, iron, and total acid number (TAN) in the crude oil fractions negatively impact these downstream processes, and others, including hydrotreating, hydrocracking and FCC to name just a few. These contaminants are present in the crude oil fractions in various organic hydrocarbon molecules and in various concentrations.
Many heteroatom contaminants appear in hydrocarbon streams in both oxidized and unoxidized form. For example, sulfur can appear in oxidized form as, for example, a sulfoxide, sulfone, or sulfonate, or in unoxidized form as, for example, a thiophene. Naturally occurring hydrocarbon streams include both oxidized and unoxidized heteroatoms, including both oxidized and unoxidized forms of sulfur. The oxidized heteroatom content of a hydrocarbon stream may also be increased through artificial processes. FIG. 1 describes a table of available oxidation states for organic heteroatom compounds.
Sulfur is widely recognized as the most egregious heteroatom contaminant as a result of the environmental hazard caused by its release into the environment after combustion. It is believed, sulfur oxides from combustion (known collectively as SOx emissions) contribute to the formation of acid rain and also to the reduction of the efficiency of catalytic converters in automobiles. Furthermore, sulfur compounds are thought to ultimately increase the particulate content of combustion products. Nitrogen, phosphorus, and other heteroatom contaminants present similar environmental risks.
A variety of methods have been implemented for removing sulfur compounds either from fuels before combustion or from emission gases afterward. Most refineries employ hydrodesulfurization (HDS) as the predominant process for removing sulfur from hydrocarbon streams. HDS remains a cost-effective option for light streams with sulfur levels up to about 2% (w/w) elemental sulfur, but the environmental and economic benefits of HDS are offset in very heavy and sour (>2% elemental sulfur) streams because the energy input to the reaction, the high pressures and the amount of hydrogen necessary to remove the sulfur paradoxically create a substantial CO2 emission problem.
Because of these issues, reduction of contaminants and, in particular, of the sulfur content in hydrocarbon streams has become a major objective of environmental legislation worldwide. Sulfur is regulated in the United States for on-road diesel at a maximum concentration of 15 ppm. By October 2012, sulfur specifications will be 15 ppm for non-road, locomotive, and marine diesel fuel. In the European Union that specification is expected to tighten to 10 ppm in January 2011 for diesels intended for inland waterways and for on-road and off-road diesel operated equipment. In China, the on-road diesel specification will be 10 ppm by 2012. Currently the tightest specifications in the world are in Japan, where the on-road diesel specification is 10 ppm.
Refiners typically use catalytic hydrodesulfurizing (“HDS”, commonly referred to as “hydrotreating”) methods to lower the sulfur content of hydrocarbon fuels, decrease the total acid number, and increase the API gravity. In HDS, a hydrocarbon stream that is derived from petroleum distillation is treated in a reactor that operates at temperatures ranging between 575 and 750° F. (about 300 to about 400° C.), a hydrogen pressure that ranges between 430 to 14,500 psi (3000 to 10,000 kPa or 30 to 100 atmospheres) and hourly space velocities ranging between 0.5 and 4 h−1. Dibenzothiophenes in the feed react with hydrogen when in contact with a catalyst arranged in a fixed bed that comprises metal sulfides from groups VI and VIII (e.g., cobalt and molybdenum sulfides or nickel and molybdenum sulfides) supported on alumina. Because of the operating conditions and the use of hydrogen, these methods can be costly both in capital investment and operating costs.
As is currently known, HDS or hydrotreating may provide a treated product in compliance with the current strict sulfur level targets. However, due to the presence of sterically hindered refractory sulfur compounds such as substituted dibenzothiophenes, the process is not without issues. For example, it is particularly difficult to eliminate traces of sulfur using such catalytic processes when the sulfur is contained in molecules such as dibenzothiophene with alkyl substituents in position 4-, or 4- and 6-positions of the parent ring. Attempts to completely convert these species, which are more prevalent in heavier stocks such as diesel fuel and fuel oil, have resulted in increased equipment costs, more frequent catalyst replacements, degradation of product quality due to side reactions, and continued inability to comply with the strictest sulfur requirements for some feeds.
This has prompted many to pursue non-hydrogen alternatives to desulfurization, such as oxydesulfurization. One attempt at solving the problem discussed above includes selectively desulfurizing dibenzothiophenes contained in the hydrocarbon stream by oxidizing the dibenzothiophenes into a sulfone in the presence of an oxidizing agent, followed by optionally separating the sulfone compounds from the rest of the hydrocarbon stream and further reacting the sulfones with a caustic to remove the sulfur moiety from the hydrocarbon fragment.
Oxidation has been found to be beneficial because oxidized sulfur compounds can be removed using a variety of separation processes that rely on the altered chemical properties such as the solubility, volatility, and reactivity of the sulfone compounds. An important consideration in employing oxidation is chemical selectivity. Selective oxidation of sulfur heteroatom moieties without oxidizing the plethora of olefins and benzylic hydrocarbons found in crude oils, refinery intermediates, and refinery products remains a significant challenge. One selective sulfoxidation method and system is disclosed in International Publication Number WO 2009/120238 A1, to Litz et al. The inventors of the present disclosure have further discovered that the catalyst of the above-mentioned international publication number is further capable of oxidizing additional heteroatoms, including, but not limited to nitrogen and phosphorus found as naturally abundant contaminants in crude oils, refinery intermediates, and refinery products as organic heteroatom-containing compounds. FIG. 1 describes a table of available oxidation states for organic heteroatom compounds.
Another concern with heteroatom oxidation lies in the fate of the oxidized organic heteroatom compounds produced. If the oxidized organic heteroatom compounds are hydrotreated, they may be converted back to the original heteroatom compounds thereby regenerating the original problem. The feed heteroatom content may be likely to be in the range of 0% to 10% by weight heteroatom. Heteroatoms, on average, comprise about 15 wt % of substituted and unsubstituted organic heteroatom molecules. Therefore, up to 67 wt % of the oil may be removed as oxidized organic heteroatom extract if not removed from the organic molecules. For a typical refinery processing 40,000 barrels per day of crude oil, up to 27,000 barrels per day of oxidized organic heteroatom oil will be generated, which is believed to be too much to dispose conventionally as a waste product. Further, the disposal of oxidized organic heteroatom oil also wastes valuable hydrocarbons, which could theoretically be recycled if an efficient process were available.
A considerable challenge presented to heteroatom removal remains the removal of the oxidized heteroatom fragment from the oxidized organic heteroatom compounds found in naturally occurring hydrocarbons or created by oxidation of the initial organic heteroatom species, without producing substantial oxygenated by-product. Therefore, a need exists for methods and systems for upgrading heteroatom-contaminated hydrocarbon feed streams by removing heteroatom contaminants from hydrocarbon streams with the added benefit of decreasing the total acid number and increasing the API gravity of the resulting product relative to the contaminated hydrocarbon feed stream.